MEMS micromirrors have long been used for steering light beams in a variety of applications such as bar code scanners, image projectors, and optical networking. In all such applications, the micromirror element is actuated in response to an external stimulus provided by a controlling mechanism. One of the well-established and most-widely used methods of actuating the micromirrors is by electrostatic means. See, e.g., U.S. Pat. Nos. 4,317,611, 5,212,582, and 6,028,689. In this method a drive electrode is electrically isolated from but placed in close proximity to a micromirror. The micromirror body is biased at a certain voltage potential and the drive electrode is biased at another potential level. The difference between the potential levels exerts an electrostatic force on the micromirror element and changes its position.
Micromirrors can be controlled in digital mode or in analog mode, depending on the requirements of the particular application. Digital control mode is shown, e.g., in U.S. Pat. No. 5,535,047, where the micromirror element is in one of the two stable positions in response to a digital control signal.
On the other hand, applications that need positioning of the mirror element at arbitrary intermediate points of its overall movement range require analog control mode of operation. See, e.g., U.S. Pat. Nos. 4,441,791 and 6,028,689. The analog control mode is typically complemented by a closed control loop that measures the position of the micromirror element and provides corrective feedback to the driving circuitry.
It is desirable to build large arrays of micromirrors in a variety of applications. One such application is a spatial light modulator where the mirror array is used to reproduce an image on a projection screen by selectively steering beams from a uniform light source that shines on the array. Another application of the micromirror arrays is in the area of optical networking where individual light beams carrying digital data are steered by the micromirrors of the array for traffic routing in an “all optical” network. In the latter application, the size of the array that can feasibly be attained is an important parameter since it defines the volume of network traffic that an optical router can handle.
One of the problems associated with providing electrostatic drive to a large array of micromirrors is the prohibitively large number of control lines that are required to access the drive electrodes. A solution to this problem is a matrix addressing scheme such as the one disclosed in U.S. Pat. No. 4,441,791, where analog voltage levels are written to capacitors that serve as storage elements in a time multiplexed arrangement. A similar addressing scheme is also described in U.S. Pat. No. 4,271,488. Provided that the leakage currents are low and the update rate of the system can accommodate the oversampling level required by the control loop, the voltages on these holding capacitors can bias the drive electrodes. This method works satisfactorily when the accuracy requirement of the system is low, and the excitement voltage levels that the micromirrors require are suitable for making relatively simple analog switches. These analog switches may be integrated along with the micromirrors or integrated separately and assembled at a later stage in close proximity with the micromirrors.
However, when high voltages (e.g., defined as voltage levels on the order of a few tens of Volts to hundreds of Volts) and high accuracy levels (˜14 bits) are required to excite the micromirrors, this approach is no longer suitable since it is difficult to make analog switches that can switch large voltage levels while satisfying various accuracy parameters such as off-isolation, crosstalk, and charge injection. Even if an ideal switch could be built and integrated within the array, the resulting system would require considerable external resources in digital-to-analog converters (DACs) and high voltage high speed voltage buffers for setting up and delivering the precise analog voltage levels to the array. Moreover, these external resources would need to meet severe performance requirements. These constraints impose substantial limitations on the size of the micromirror arrays that can be built.
Therefore, there is a need for electrostatic drivers for micromirror arrays that do not require high voltage analog switches or closely packed analog transmission lines.